1. Field of the Invention
The present invention relates to a superconducting device and a method for manufacturing the superconducting device. More specifically, the present invention relates to a superconducting device formed of an thin film of oxide superconductor material formed on a substrate and a insulator thin film formed on the oxide superconductor material thin film, and a method for manufacturing the superconducting device.
2. Description of Related Art
In the case of using an oxide superconductor material in a superconducting device, it it necessary to form and stack a thin film of oxide superconductor material, and then, to form a thin film made of, for example, an insulator or a semiconductor, on the thin film of oxide superconductor material. For example, when a superconductor/insulator/superconductor junction called a "tunnel type" Josephson junction is formed by using an oxide superconductor material, it is necessary to sequentially stack a first thin film of oxide superconductor material, a thin film of insulator material, and a second thin film of oxide superconductor material in the named order.
In this tunnel type Josephson junction, the thickness of the insulator material thin film is generally determined by a coherence length of the superconductor material. Since the oxide superconductor materials have a very short coherence length, it is necessary to utilize the thin film of non-superconductor material having the thickness on the order of a few nanometers.
However, if the thin film of oxide superconductor material is exposed to air, both superconductivity and crystallinity thereof will be deteriorated to the extent of about 1 nm depth from its surface. In ordinary cases, when a second thin film is deposited on the thin film of oxide superconductor material, a deposition apparatus different from that used for formation of the thin film of oxide superconductor material is used, and therefore, the thin film of oxide superconductor material is inevitably exposed to air in the course of feeding from one deposition apparatus to another deposition apparatus. Because of this, it has been a conventional practice to heat the thin film of oxide superconductor material at about 700.degree. C. under an ultra high vacuum on the order of 1.times.10.sup.-9 Torr, before the second thin film is deposited on the thin film of oxide superconductor material.
The thin film of oxide superconductor material subjected to the above mentioned heat-treatment can have a surface of an improved crystallinity, and in addition, it is possible to epitaxially grow the second thin film on the first thin film of oxide superconductor material. However, the heating of the thin film of oxide superconductor material under the above mentioned ultra high vacuum will result in loss of oxygen from the thin film of oxide superconductor material crystal, which will deteriorate or lose the superconduction characteristics.
On the other hand, if the heat treatment is performed in the oxygen atmosphere, no deterioration in superconduction characteristics of the thin film of oxide superconductor material will occur, but the crystallinity of the thin film surface is not improved.